1. Technical Field
The present invention relates to the field of focus dose metrology, and more particularly, to measurement methods and target which improve focus and dose measurement accuracy and flexibility.
2. Discussion of Related Art
In Signal Response Metrology (SRM), the responses of signals are learned as the function of a few parameters of interest. Using a set of designated experiments, the parameters of interest are changed and the signals are measured. In the case of lithographic printing tool (such as scanner or a stepper) the focus and dose metrology are done using SRM with the focus and dose as parameters. A set of cells (i.e., a metrology target) is printed for different focus and dose values on a special wafer, namely the Focus Exposure Matrix (FEM) wafer. The learned signals may be derived by a corresponding metrology tool, e.g., implementing angle resolved scatterometry technology.
The inherent problems in SRM are the errors in the learned signals. In the case of focus metrology, the ambiguity in the FEM focus can be about 20 nm. These focus errors are expected on both the FEM and production wafers. Such a large ambiguity in scanner focus positions on FEM wafer doesn't allow creating a well-calibrated model and causes large errors in subsequent focus\dose measurements. Moreover, process variations, as in under-layers and resist, give rise to differences between the FEM and production printed patterns, and in turn of the measured signals.
Prior art focus dose measurements typically use two cells consisting of line-space patterns. One cell, termed Dense cell, has lines at a minimal pitch, and the other cell, termed ISO cell, has the same lines as the Dense cell but with a double pitch. While the ISO cell is very sensitive to both focus and dose the Dense cell is insensitive to focus. Hence, it is possible to de-correlate the focus from the dose contribution (see Mack, Chris 2008, Fundamental principles of optical lithography: the science of microfabrication, John Wiley & Sons). Asymmetric targets have asymmetric feature(s) which depend on focus and exposure. Such targets typically have a large pitch (400-900 nm), which enables to measure the differential signal between the first orders. Multiple targets with different pitch and asymmetry are used to de-correlate focus and dose (see WIPO Publication No. 2013/189724, which is incorporated herein by reference in its entirety).